1. Field of the Invention
The present invention relates to an image forming device such as a color laser printer.
2. Description of the Related Art
A tandem laser printer is one type of laser printer for forming full-color images. One example of a tandem laser printer includes an image forming unit for each color. Each image forming unit includes a developing roller, a photosensitive drum, a charge unit, and an exposure unit. The developing roller, the charge unit, and the exposure unit are disposed in confrontation with the photosensitive drum. The charge unit forms a uniform charge on the surface of the photosensitive drum. The exposure unit selectively exposes portions of the charged surface to form a latent static-electric image on the surface of the photosensitive drum. The developing roller bears toner on its surface and develops the latent static-electric image using the toner.
The visible toner images developed for each color are transferred one at a time in order onto a transfer belt so that a full-color image can be formed at substantially the same speed as a monochrome image.
Some tandem color laser printers can selectively switch between forming color images and monochrome images. That is, to form a color image, all four photosensitive drums are driven to form images in the four colors of yellow, magenta, cyan, and black. The different color images are transferred one at a time into a stacked condition onto a transfer belt to form a color image. On the other hand, to form a monochrome image, only the photosensitive drum for forming black image is driven so that only a black image is transferred onto the transfer belt to form a monochrome image.
Four motors are provided, one for each photosensitive drum in order to enable selective switching between multi-color and monochrome image formation. All four of the motors are driven when a multi-color image is to be formed and only the motor that corresponds to the black photosensitive drum is driven when a monochrome image is to be formed. However, providing four motors in this manner increases production costs. Also, the control circuit must be able to control drive of all the motors, which increases the complexity of the printer.
It is conceivable to drive all four photosensitive drums using a single motor in order to reduce production costs and simplify configuration. To achieve this, it is conceivable to provide an electromagnetic clutch between the single motor and the photosensitive drums that can be switched to selectively transmit drive force from the motor to one or all of the four photosensitive drums. Monochrome images can be formed when only one of the photosensitive drum is driven and multi-color images can be formed when all four photosensitive drums are driven. By providing this electromagnetic clutch, there is no need to provide a separate motor for all of the four photosensitive drums.
However, with this conceivable configuration, the electromagnetic clutch itself as well as circuitry for controlling the switching operation of the electromagnetic clutch must be provided, thereby increasing production costs and complexity of the printer. Also, a large torque is required to rotate all four of the photosensitive drums. As a result, a great deal of power would be required to prevent the electromagnetic clutch from slipping while a multi-color image is being formed. This would greatly increase running costs.
It is an objective of the present invention to overcome the above-described problems and to provide an image forming device with low production coats and a simple configuration capable of selectively switching drive of a plurality of photosensitive bodies and selectively forming multi-color and monochrome images.
To achieve the above-described objectives, an image forming device according to one aspect of the present invention includes a plurality of photosensitive bodies and a single drive unit. The plurality of photosensitive bodies each forms an image having a different color. The single drive unit switches between driving at least one of the photosensitive bodies and at least a different one of the photosensitive bodies.
An image forming device according to another aspect of the present invention includes a plurality of developing units, a plurality of photosensitive bodies, a transfer unit, a drive unit, and a transmission mechanism. Each of the developing units is provided for a different one of a plurality of colors. The photosensitive bodies are provided in correspondence with the developing units. The transfer unit is disposed in confrontation with the photosensitive bodies. The drive unit generates drive force. The transmission mechanism switches transmission of the drive force from the drive unit to photosensitive bodies selected in accordance with drive condition of the drive unit.
According to still another aspect of the present invention, an image forming device includes a plurality of developing units, a plurality of photosensitive bodies, a transfer unit, a drive unit, and a transmission mechanism. Each developing unit is provided for a different one of a plurality of colors. The photosensitive bodies are provided in correspondence with the developing units. The transfer unit is disposed in confrontation with the photosensitive bodies. The drive unit switchingly generates forward drive force and reverse drive force. The transmission mechanism transmits drive force from the drive unit to the photosensitive bodies. The transmission mechanism transmits the same direction of drive force to the photosensitive bodies regardless of whether the drive unit generates forward drive force or reverse drive force.